Pin the Rust toolchain and scope tokio features

[CMGS]

What

• rust-toolchain.toml pins 1.98.0 instead of stable; the dtolnay/rust-toolchain@stable steps are gone from ci.yml and release.yml, so the pin is the single source of truth for gates, release builds and the rust:1 image build (the same shape as sandbox's silkd/boot workflows).
• tokio enables only the features the workspace uses (rt-multi-thread, macros, time, sync, net, signal, io-util) instead of full; Cargo.lock drops the one crate full pulled in.
• Two clippy 1.98 findings the pin surfaces: state/avail.rs sums the MGET pairs with as_chunks::<2>() (chunks_exact with a constant size is now rejected), and result_large_err now covers async fns, so the five async view helpers returning Result<_, Response> carry the same per-site allow their sync siblings already had. A floating stable would have hit both as soon as the runners moved to 1.98.

Evidence (local, arm64)

• cargo fmt --all -- --check: clean
• cargo clippy --workspace --all-targets -- -D warnings: clean on 1.98.0
• cargo test --workspace: 28 suites, 548 passed, 0 failed, 3 ignored (Postgres/Redis-gated suites not run locally); gw-state and gw-views re-run after the lint changes
• docker build --platform linux/arm64 .: rustup inside the rust:1 builder synced 1.98.0-aarch64-unknown-linux-gnu before compiling, so the image no longer depends on what rust:1 floats to